THE RESPIRATORY DEVELOPMENT OF ATLANTIC SALMON .2. PARTITIONING OF OXYGEN-UPTAKE AMONG GILLS, YOLK-SAC AND BODY SURFACES

During post-hatch development of Atlantic salmon (Salmo salar L.), O-2 uptake partitioning changes from primarily cutaneous to primarily bra nchial. Over 80 % of post-hatch O-2 uptake was cutaneous, with the yol k sac responsible for 33 % of total O-2 uptake,.The well-vascularized yolk sac was a less effective gas exchanger than the unperfused skin o f the body, suggesting that oxygen delivery is by direct diffusion to the tissues. Branchial O-2 uptake increased quickly as gill lamellae d eveloped, contributing 60 % of total O-2 uptake before the completion of yolk resorption (body mass 0.2g) and increasing to 69-81 % in fish weighing over 0.3g. The area-specific O-2 uptake of the skin decreased through development as skin thickness increased, while that of the gi lls increased from 0.10 mu g h(-1) mm(-2) to 0.23 mu g h(-1) mm(-2). P artitioning of O-2 uptake of the skin and gills changed in concert wit h changes in the partitioning of the anatomical diffusion factor (ADF, mass-specific surface area per unit diffusion distance) between skin and gills, which changed from more than 95 % to less than 10 % cutaneo us; thus, ADF is a useful rough indicator of oxygen uptake potential. Caution should be used in predicting oxygen uptake potential from ADF, however, because O-2 uptake per unit diffusion barrier of the yolk sa c was less than half that of the general body surface, and O-2 uptake per unit diffusion barrier of the gills changed dramatically over deve lopment.